The present invention relates to data processing apparatuses which include preprogrammed functions enabling the user to compute various interest rate dependent functions. This function capability is most often found in the so called "business" calculators. These calculators generally include a wide variety of functions in addition to the four basic arithmetic operational functions of addition, subtraction, multiplication and division. These machines typically include various keystroke sequencies which enable the operator to enter the independent variables in one or more interest rate dependent functions and to designate the dependent variable to be calculated.
A problem exists with the typical calculator of this type. Most often calculators of this type have been functionally programmed to operate in only a single interest rate mode. In the United States interest rates for interest rate dependent calculations are typically specified as an annual percentage rate (APR). The annual percentage rate is also known as annual nominal interest rate. The annual percentage rate is computed by multiplying the interest per compounding period by the number of compounding periods per year. However, in other contexts the interest rate may be specified in other modes. One of these other modes is annual effective interest rate (EFF) which is computed based upon compound interest and is therefore dependent upon the number of compounding periods per year. A third type of interest rate sometimes employed is annual continuous interest rate (CONT). An annual continuous interest rate is based upon the concept of continuous compounding and forms the limit of annual effective interest rate as the number of compounding periods per year becomes very large. Previous business calculator design has heretofore not permitted calculation of interest rate dependent formulas in more than one interest rate mode. Thus, the operator is required to recall and correctly execute key strokes for calculating the interest rate dependent functions in the differing modes and for calculating the interest rate mode conversions, rather than relying upon the preprogrammed functions of the machine. Such a requirement places a greater burden on the operator to correctly recall the function to be performed, and to correctly depress the key sequence required to perform this function. This requires greater operator attention and leads to an enhanced probability of operator errors in the performance of the required calculation. In addition, even assuming that the operator correctly remembers and correctly enters the proper sequence, this operation requires greater time than the operation of a preprogrammed function which typically requires depression of only one or two keys of the keyboard.